 A cascaded grating coupling scheme for unidirectionally launching surface plasmids from free space light with ultra-broadband and wide-angle efficiency is proposed, theoretically investigated, and applied in sensing. The cascaded sub-gratings of different periods couple incident light into surface plasmid polaritons, SPPs, which interact with the sensed medium and are coupled back into freely propagating light through a nanoslit in the metal film. Broadband sensing is achieved by observing changes in SPP mediated light coupled out through the slit, and a quantitative theoretical model is developed for performance prediction. Proof of principal demonstrations show spectral full width half maximum of 400 nanometers and angular width of up to 25 degrees, wavelength sensitivity as high as 852 nanometers RU-1, and a relative intensity change up to 5 times 10 4% RU-1. This article was authored by Cohen Lee, Feng Xiaoh, Fan Lu, and others.